The application of megasonic energy to substrate wet processing has become widely accepted, especially in semiconductor fabrication. As device/feature size has shrunk, and as substrate structures have become more vulnerable to damage, the frequency of megasonic systems have increased and have trended towards the megahertz range.
The application of sonic energy having frequencies approaching and exceeding one megahertz are often referred to as megasonic processing. These higher frequencies are used in an attempt to dislodge smaller contaminant particles and to reduce the localized energy release associated with bubble formation/collapse (cavitation and possible microcavitation) that some have theorized can lead to substrate damage as has been observed with lower frequency ultrasonic cleaners.
Historically, batch-processing systems have been designed to introduce megasonic energy parallel to substrate surfaces. In the quest for faster processing, particularly with the push for single wafer processing, megasonic designs have tended towards high-energy systems with introduction of energy perpendicular to substrate surfaces. This has led to concerns regarding megasonic damage to sensitive structures.
Therefore it is desirable to find gentler ways of applying megasonic energy that will rapidly remove ever-smaller contaminants.